The silicon (Si) integrated circuit (IC) has dominated electronics and has helped it grow to become one of the world's largest and most critical industries over the past thirty-five years. However, because of a combination of physical and economic reasons, the miniaturization that has accompanied the growth of Si ICs is reaching its limit. The present scale of devices is on the order of tenths of micrometers. New solutions are being proposed to take electronics to ever smaller levels; such current solutions are directed to constructing nanometer scale devices.
Prior proposed solutions to the problem of constructing nanometer scale devices have involved (1) the utilization of extremely fine scale lithography using X-rays, electrons, ions, scanning probes, or stamping to define the device components; (2) direct writing of the device components by electrons, ions, or scanning probes; or (3) the direct chemical synthesis and linking of components with covalent bonds. The major problem with (1) is that the wafer on which the devices are built must be aligned to within a small fraction of the size of the device features in at least two dimensions for several successive stages of lithography, followed by etching or deposition to build the devices. This level of control does not scale well as device sizes are reduced to nanometer scale dimensions. It becomes extremely expensive to implement as devices are scaled down to nanometer scale dimensions. The major problem with (2) is that it is a serial process, and direct writing a wafer full of complex devices, each containing trillions of components, could well require many years. Finally, the problem with (3) is that high information content molecules are typically macromolecular structures such as proteins and DNA, and both have extremely complex and, to date, unpredictable secondary and tertiary structures that cause them to twist into helices, fold into sheets, and form other complex 3D structures that will have a significant and usually deleterious effect on their desired electrical properties as well as make interfacing them to the outside world impossible.
The present inventors have developed new approaches to nanometer-scale devices, comprising crossed nano-scale wires that are joined at their intersecting junctions with bi-stable molecules, as disclosed and claimed in above-referenced U.S. Pat. No. 6,459,095. Wires, such as silicon, carbon and/or metal, are formed in two-dimensional arrays. A bi-stable molecule, such as rotaxane or pseudorotaxane, is formed at each intersection of a pair of wires. The bi-stable molecule is switchable between two states upon application of a voltage along a selected pair of wires.
Prior solutions to the problem of constructing a nanometer scale transistor (a three-terminal device with gain) involve the precise positioning of three or four components within a nanometer. A proposed prior solution is to position a quantum dot between two wires, which act as the source and drain of the transistor, in tunneling contact with the quantum dot (this is known as a single-electron transistor, or SET, and was originally proposed by K. Likharev); see, e.g., K. K. Likharev, “Correlated discrete transfer of single electrons in ultrasmall tunnel junctions”, IBM Journal of Research and Development, Vol. 32, pp. 144–158 (January 1998). A third wire is positioned in capacitive contact with the dot, which is the gate. The voltage on the gate changes the energy levels in the quantum dot, which creates a coulomb blockade to current flowing from the source to the drain.
The problem with the foregoing construction is that a total of three wires plus a quantum dot have to be precisely positioned. This precision of placement must clearly be better than the sizes of the components being placed, which are in the order of nanometers. Such devices have been fabricated using conventional lithography, in which case they are so large they must be operated at very low temperatures (near absolute zero). They have also been fabricated using electron beam lithography and utilizing scanning probe microscopes as direct-write tools.
Thus, there remains a need to provide a nanoscale transistor that affords the advantages of prior art devices, while avoiding most, if not all, their disadvantages.